Mat-immobilized-electrolyte type, Pb-acid electric storage batteries include at least one galvanic cell element comprising opposite polarity electrodes (e.g., a stack of positive and negative plates) separated one from the other by a fibrous, absorbent mat (e.g., polypropylene, Dynel, glass, or felt). The mat in its uncompressed state is thicker than the gap between adjacent opposite polarity electrodes and is compressed between the electrodes where it capillarily immobilizes the battery's electrolyte within the cell element. One such battery is described in U.S. Pat. No. 3,553,020 filed Dec. 20, 1968 in the names of Corbin et al and assigned to the assignee of the present invention.
Mat-immobilized-electrolyte batteries may be either of the "flooded" electrolyte type or "starved" electrolyte type. Flooded-electrolyte-type batteries have the mat and electrodes saturated with electrolyte in that the volume of the electrolyte substantially equals or exceeds the void volume of the pores within the mat and the electrodes. Starved-electrolyte-type batteries, on the other hand, have less electrolyte volume than the void volume of the pores within the mat and the electrodes. So-called gas-recombinant batteries, for example, operate in a starved electrolyte condition wherein electrolyte volume is only about 60% to 90% of the available void volume within the mat and electrodes and thereby provides sufficient void space therein to permit migration of oxygen from the positive electrode directly to the negative electrode for consumption thereat. One such gas-recombinant battery employing a resilient, glass fiber mat is described in McClelland et al U.S. Pat. No. 3,862,861.
Assembly and acid-filling of mat-immobilized electrolyte batteries is more difficult than conventional batteries which do not have compressed fibrous mats between the plates. Assembly of mat-immobilized electrolyte batteries, for example, requires the extra step (and associated equipment) of compressing the stack of plates and mats in order to place the stack in the container. Conventional (i.e., mat-free) lead-acid storage batteries are commonly filled with sulfuric acid electrolyte by placing the cell elements in the battery container in the unformed (i.e., uncharged condition) state wherein the positive and negative electrodes both comprise essentially lead oxide. Thereafter, H.sub.2 SO.sub.4 forming acid is metered into the cell compartment atop the elements until the compartment is filled. The electrodes are subsequently electrolytically formed (i.e., initially charged) by passing a forming current therethrough. In some cases, the residual forming acid is then dumped and fresh acid substituted therefor as the working electrolyte. In other instances, the forming acid has sufficient residual strength to remain in the battery as the working electrolyte. Between the time the forming acid is added and the formation process begins, the sulfuric acid reacts exothermally with the lead oxide in the positive and negative electrodes to form lead sulfate in both the positive and negative electrodes. This sulfation reaction is known in the art as "pickling". The pickling reaction is initially quite vigorous and results in the generation of considerable heat and some gas. This initial pickling reaction subsides considerably after the first several minutes following acid addition as the acid weakens (i.e., becomes more dilute), the electrodes become sulfated and the lead oxide available for reaction decreases.
Electrolytic formation of the battery follows pickling and involves passing forming current through the battery to convert the lead sulfate in the electrodes to lead dioxide in the positive electrodes, lead in the negative electrodes and to reconstitute sulfuric acid in the electrolyte. In addition to any residual pickling heat, the electrolytic formation process adds considerable I.sup.2 R heat to the battery thereby causing the temperature of the elements to increase significantly. High element temperatures during formation cause the gassing overvoltage of the electrodes to drop which in turn undesirably causes excessive gassing. Batteries containing glass mats compressed between the electrodes interfere with the circulation of electrolyte between the plates and the escape of heat and gases from the cell element and accordingly retain the pickling heat for significantly longer periods of time than conventional mat-free batteries as well as trap gases within the cell element.
In conventional, mat-free batteries the gassing that occurs during pickling and formation, while undesirable, can nonetheless be tolerated as the gas can readily escape the element from between the plates. For batteries having glass mats compressed between the plates, however, gassing during formation must be kept to a minimum in order to prevent any gas from becoming entrapped within the mat causing so-called "dry spots" therein where little or no acid is present. In this regard, dry spots, whether formed during the pickling or formation reactions, will not only interfere with the formation reaction by causing incomplete formation in some regions of the electrodes and increased current densities in other regions thereof, but ultimately results in undesirable lead treeing (i.e., dendrite growth) between the plates, reduced battery capacity and cold cranking performance, as well as shortened cycle life. Moreover, in conventional, mat-free batteries the electrolyte is more mobile and free to mix and circulate within the cell element due to natural convection so that formation of the plates' active material occurs substantially uniformly throughout the cell element.
Filling mat-immobilized-electrolyte type batteries with electrolyte by dispensing the electrolyte atop the cell element after the cell elements have been placed in their container (i.e., the way conventional batterial are typically filled) can have a detrimental affect on the performance of the battery. In this regard, when so filled, the mats often: make it difficult to obtain uniform distribution of acid throughout the cell element; prevent mixing/circulation of the electrolyte within the cell element during pickling and formation; can result in vaporization of the electrolyte within the cell element during pickling; and/or result in mats containing pockets of trapped gases (i.e., dry spots). More specifically, electrolyte introduced into the container atop the cell element percolates down through the cell element from the top thereof at a rate limited by the wicking rate of the glass mat. The presence of the mat can cause trapping of air within the element, can prevent the ready escape of the gases and heat generated therein during the pickling reaction and can prevent acid from circulating between the electrodes. Some of the filling acid may run down the sides of the element and make inroads into the elements from the sides, but this too is slow and still traps gases and heat. As a result, not only are dry spots prevalent, but the temperature of the element is not uniform and may be elevated to an undesirably high level and maintained thereat for a prolonged period of time. Moreover, the concentration of the electrolyte within the cell element tends to vary from one location to the next. One reason for this is the stratification that occurs by virtue of the electrolyte front's descending down or otherwise into the cell element. As the electrolyte front (i.e., the initial few centimeters of the electrolyte wave moving into the element) advances into the element, it is more rapidly depleted of its H.sub.2 SO.sub.4 content then is the electrolyte tracking behind the front. As a result, by the time the liquid front moves into the center of the element, it has much lower sulfuric acid concentration than the acid tracking behind it (e.g., near top of the element). If the wicking rate and starting acid temperature are slow and high (e.g., ambient temperature) respectively, it is possible to end up with a slightly alkaline aqueous solution in the middle of the element. This results in high Pb.sup.++ solubility due to the high pH and high temperatures. If this solubility is high and remains high even for only several minutes, the soluble lead migrates into the separators where it is converted to lead which electrically bridges (i.e., shorts) adjacent electrodes during formation and/or subsequent charging. Moreover, this acid concentration imbalance affects the conductivity of the electrolyte at different locations in the element which, in turn, affects current density distribution during formation. Another reason for electrolyte concentration variations is the formation of dry spots resulting from trapped gases discussed above. Eventually, acid will infiltrate even into the dry spots, at least to some extent, but is quickly consumed by the unformed PbO in the regions of the electrodes adjacent the dry spots resulting in pockets of low concentration acid within the element. Finally, because mat-immobilized electrolyte elements do not take up electrolyte as quickly as mat-free elements, care must be taken to meter the electrolyte into the container atop the element at a sufficiently slow rate as to preclude overflow thereof from the top of the container above the cell element.
McCartney, Jr. et al U.S. Pat. No. 4,743,270 proposes to minimize some of the aforesaid problems by putting the electrolyte into the container first, and then immersing the cell element slowly into the electrolyte. Another manufacturer reduces the need to compress the stack during assembly into its container and purportedly reduces the acid-fill time by impregnating the resilient fibrous glass mat with a water/acid soluble glue/binder and precompressing the mat by as much as 50 percent. The binder holds the separator in its precompressed state until it is wetted by electrolyte during filling. The glue dissolves as soon as it is wetted by electrolyte and allows the resilient glass mat to immediately spring back, swell within the cell and press tightly against the plates it is sandwiched between. Suggested water soluble binders include methyl cellulose (preferred), carboxymethy cellulose, ethylhydroxyethyl cellulose, hydroxyethyl cellulose, fish glue, soybean glue, guar flour (from the fruit of the carob tree) as well as starch and dextrin based glues.
While the use of precompressed mats bound with water/acid-soluble binders facilitates assembly and perhaps accelerates acid filling to some extent, they do not completely solve the acid filling problem much less the heat and gas trapping or electrolyte circulation problems discussed above in connection with pickling and formation. In this regard, the mats bound only by water/acid soluble binders begin to swell as soon as they are wetted with electrolyte so that the first wetted portions of the mat (i.e., adjacent the electrolyte fill opening in the container) swell, partially close off the acid entry zone and retard rapid flow and even distribution of the electrolyte throughout the cell element. This is true even though a substantial portion of the separator which is remote from the fill opening might still be compressed.
A better solution to the aforesaid filling, pickling and formation problems would be (1) to get the electrolyte distributed evenly throughout a mat-immobilized electrolyte-type cell element as quickly as possible, (2) to insure that the gases and heat generated during pickling and formation can readily escape the cell element, and (3) to maintain good mixing/circulation of the electrolyte in the element at least into the early stages of the formation cycle.